A process for producing semiconductor wafers consists primarily of a step of pulling us a single-crystal for producing single-crystal ingots and a step of processing the single-crystal ingots thus produced. This processing step typically includes a slicing step, a lapping step, a chamfering step, an etching step, a mirror polishing step, a cleaning step and the like, and semiconductor wafers with mirror polished surfaces are produced through these steps.
In the mirror polishing step, several stages of polishing steps including rough polishing, finish polishing and the like are performed. In the finish-polishing step, for example, one side of the wafer is subjected to chemical mechanical polishing (CMP) with a single-side polishing apparatus 200 as shown in FIG. 6. CMP is a technique in which a polishing solution having an etching effect on the wafer, the work being polished, is used to etch the wafer while mechanically polishing the wafer with abrasive grains contained in the polishing solution. The single-side polishing apparatus 200 includes a head 202 for retaining a wafer and a polishing plate 210 having a polishing cloth 212 provided on its surface. The head 202 presses a polishing target surface of the wafer against the polishing cloth 212. Then, the wafer surface is polished by rotating the head 202 together with the polishing plate 210 while a polishing solution 228 is supplied from a polishing solution supply unit 226 to upon the polishing cloth 212.
Typically, various impurities become adhered to an unused polishing cloth in the process of production of the polishing cloth. These impurities cause damage to the wafer surface. For this reason, large numbers of light point defects (LPDs) are detected in a post-polishing inspection step performed on the surface of a wafer polished with a polishing cloth immediately after the start of use of the cloth. To solve this problem, as described in JP 2005-209863 A (PTL 1), in the case of using a new polishing cloth, it is used to polish a stipulated number of wafers not to be shipped as products and then the polishing of wafers to be shipped as products is performed. Note that in this Specification, the initial polishing step which is performed immediately after the start of use of a new polishing cloth and which provides a polished water that is not used as a product is called “dummy polishing mode,” while the polishing step which is performed thereafter and which provides a polished wafer that is used as a product is called “production polishing mode.”
Up until now, once a stipulated number of rounds of dummy polishing have been done, or in other words, once the cumulative number of wafers polished with a new polishing cloth has reached the stipulated number of wafers, a shift to production polishing mode is done, and it is typical for the “stipulated number of rounds” and “stipulated number of wafers” to be uniformly set to fixed values as long as the same kind of polishing cloth is used.